1. The Field of the Invention
The present invention relates to a lead and arsenic free and preferably gadolinium free and fluorine free optical lanthanum borate glass, to the use of such a glass in the fields of mapping, projection, telecommunication, optical communication engineering, mobile drive and laser technology, as well as to optical elements respectively preforms of such optical elements.
2. Description of Related Art
In the recent years, the tendency on the market in the field of optical technologies as well as opto-electronic technologies (application fields mapping, projection, telecommunication, optical communication engineering, mobile drive and laser technology) goes more and more into the direction of miniaturization. This can be seen with the finished products which become smaller and smaller and naturally requires an increasing miniaturization of the single structural members and components of such finished products. For the producers of optical glasses, this development means a clear decrease of the demanded volumes of rough glass in spite of increasing quantities of finished products. At the same time, there is an increasing pricing pressure from the side of the reprocessors to the producers of glass, since with the production of such smaller components made of block and/or ingot glass noticeably more waste will be produced proportionally based on the product and for the processing of such miniature parts a higher operating expense is necessary than for larger structural members.
Instead of the removing of glass portions for optical components from block or ingot glass which is common till today, therefore recently production procedures become important in which directly after the glass melt preforms respectively pills which are as close as possible to the final contour respectively geometry such as e.g. gobs or spheres may be yielded. For example, the reprocessors' requests for preforms which are close to the final geometry for re-pressing, so-called “precision gobs”, are increasing. Normally, these “precision gobs” preferably mean completely fire-polished, free or half-free formed glass portions which are already portioned and have a geometry which is close to the final form of the optical component.
Such “precision gobs” may preferably also be converted into optical elements such as lenses, aspheres etc. by the so-called “precise pressing” or “precise molding” process. Then, a further processing of the geometric form or the surface with e.g. a surface polish is no longer required. This procedure can comply with the smaller volumes of melted glass (distributed on a high number of small parts of material) in a flexible way by shorter set-up times. Because of the relatively lower number of parts per time unit and the normally smaller geometries, the creation of value cannot be caused by the value of the material alone. Rather, the products have to leave the press in a state ready for installation, i.e. laborious post-processing, cooling and/or cold re-processing must not be necessary. Because of the required high accuracy of geometries, precision instruments with high grade and therefore expensive mold materials have to be used for such a pressing procedure. The lifetimes of such molds massively affect the profitability of the products and/or materials produced. A very important factor for a long lifetime of the molds is a working temperature which is as low as possible, but which can only be lowered to a point at which the viscosity of the materials to be pressed is yet sufficient for the pressing procedure. This means, that there is a direct relationship between the processing temperature and therewith the transformation temperature Tg of a glass to be processed and the profitability of such a pressing process: The lower the transformation temperature of the glass, the longer the lifetimes of the molds; and therefore the higher the earnings. Thus, there is a demand for so-called “low-Tg-glasses”, i.e. glasses having low melting points and transformation temperatures, i.e. glasses with a viscosity at temperatures which are as low as possible which is sufficient for processing.
Further, from a process technical point of view of the melt there is a growing demand for “short” glasses, i.e. glasses having a viscosity which varies strongly within a certain viscosity range at a relatively small change in temperature. This behaviour has the advantage in the melting process that the times of hot forming, i.e. the closure times of the molds, can be decreased. Because of that, on the one hand the throughput will be increased, i.e. the cycle times will be reduced. On the other hand, because of that also the mold material will be protected which also has a positive effect on the total production costs, as described above. Such “short” glasses have the further advantage that also glasses with higher tendency to crystallization may be processed by the faster cooling than with corresponding longer glasses. Therewith prenucleation which could cause problems in succeeding steps of secondary hot forming will be avoided. This presents the possibility that such glasses may also be stretched to fibres.
Furthermore it is also desirable that, besides the mentioned and the required optical properties, the glasses are sufficiently chemically resistant.
The prior art already describes glasses with similar optical state or with a comparable chemical composition, but these glasses have immense disadvantages. In particular, many of the glasses contain higher proportions of Gd2O3 which as a rare-earth oxide has a weak band at 590 nm and thus deteriorates the internal transmittance, and/or components which increase the tendency to crystallization, such as e.g. TiO2.
US 2003/0211929 relates to an optical glass for precisely pressed products having a Tg of lower than 630° C. The latter property is achieved by the addition of a very high amount of B2O3 and ZnO. In every case, the glass contains Gd2O3 in a proportion of at least 5% by mol.
JP 2003/201142 describes an optical glass for precisely pressed products having also a low Tg. Here also this property is achieved by the addition of a very high amount of B2O3 and ZnO. In every case, the glass contains Gd2O3 in a proportion of at least 6% by weight.
US 2003/0191008 comprises an optical glass with a high refractive index for the precise pressing technology. The glass contains very high proportions of Nb2O3 of at least 30% by weight. Nb2O5 in such high proportions deteriorates the internal transmittance of the glass.
JP 2003/238198 describes an optical glass for precisely pressed products having a low Tg. The latter property is achieved by the addition of LiF and/or ZnF, wherein in every case fluorine is contained as a component in an amount of at least 9% by weight. Fluorine is disadvantageous with respect to a stable melting and production process, in particular because of the strong evaporation.
JP 2003/300751 describes a low melting glass for precisely pressed products. The low Tg of 480 to 580° C. is i.a. achieved by the addition of Bi2O3. In every case, Bi2O3 is contained and imparts self-color to the glass which deteriorates the internal transmittance of the glass.
JP 2002/173336 describes a low melting phosphate glass having a high dispersion respectively low Abbe number of 20 to 32.
DE 35 34 575 relates to a glass for eye glass lenses which in every case comprises a component for coloration. Here, lanthanum oxide is only an optional component.
DE 36 05 668 relates to an optical tellurite glass which in every case comprises toxic tellurium oxide as a component.
DE 101 26 554 describes borosilicate glasses with very high refractive indexes.
The documents EP 1 236 694 A1, US 2003/0100433 and US 2003/0211929 describe optical glasses which are lead and fluorine free, but which in every case contain Gd2O3.
JP 60-221338 relates to glasses which in every case contain lithium oxide as a component and in which at least a part of one oxide is replaced by fluorine.